1. Field of the Invention
The present invention relates to gain controllable electronic circuits and more particularly to a log-antilog gain control circuit wherein the output signal produced therefrom contains substantially no direct current component or second harmonic signal component of the fundamental frequency of the input signal.
2. Description of the Prior Art
Monolithic variable transconductance and four quadrant multiplier circuits find common use as gain control circuits. For instance, U.S. Pat. No. 3,689,752 and an article written by Walter G. Jung, "Get Gain Control of 80 to 100 dB" electronic Design 13, 21 June 74, describe such multiplier circuits. These circuits provide good gain control but may suffer from non-linearity problems if the differential transistor pairs comprising the same are not perfectly matched with respect to each other. The non-linearity problem arises from the offset voltage mismatch between the differential transistor pairs as is understood. Hence, the output signal is not an exact reproduction of the input signal.
The prior art also includes log-antilog transfer function circuits, utilizing emitter-coupled transistors, for gain control. The output collector current of the output transistor of the two emitter coupled devices is linearly related to the collector current of the input transistor device hereof with the base-collector junction being zero biased. However, prior art log-antilog circuits, although not suffering from offset induced distortion as previously described, produce second harmonic signal components of the input current in the output collector current due to the bulk resistance of the base-emitter of the output transistor. This bulk resistance caused distortion makes the transistors imperfect logarithmic devices as is generally understood. Hence, if prior art log-antilog transfer function circuits are utilized as gain control stages in audio reproduction systems, serious intermodulation problems could arise therein due to the second harmonic signal component that is generated from the fundamental audio input signal.
Therefore, a need arises for a good gain control circuit suitable for manufacture in monolithic form to overcome the problems of variable transconductance multipliers and log-antilog circuits commonly used today and which can be utilized, for an example, in high fidelity audio sound systems.